Recently, an operation speed of a computer has been dramatically improving in a fast-evolving manner, and even in the case of a personal computer intended for home use, the clock frequency thereof has reached a GHz level, thus, it is inevitable that a signal transmission speed is becoming increasingly faster. And, as an information network is gradually formed all over the society along with generalization of office automation and a LAN system, a server as peripheral equipment of the computer has become widely used because of the necessity for information management of a plurality of computers.
This server generally includes bulk memory capable of centralized management of a huge amount of information and is required to have high-speed operation performance allowing for simultaneously accessing from the plurality of computers. Therefore, the signal transmission within the server is required to be carried out faster and malfunctions during the transmission have to be minimized.
In order to achieve a use environment as described above, circuit design of a central processing unit (CPU) and performance of an IC chip for example are important, and further, circuit design of a printed wiring board on which the CPU and the IC chip are mounted also become very important. Manufacturers of the printed wiring boards have devised various means in order to achieve the enhancement of the signal transmission speed described above, and for example, a structure of the printed wiring board has been made into a multi layered structure, and as for the circuit arrangement, the circuit design is changed to decrease a signal transmission distance.
In particular, a capacitor which plays a role for stably supplying operation power of a device has generally disposed as an external layer of the printed wiring board, however, a method in which a capacitor is made of a double-sided copper clad laminate as an internal layer of a multi-layer printed wiring board has widely spread because the printed wiring board can be formed thinner and excellent performance can be obtained. And, in order to form this capacitor layer, various thin materials for forming the capacitor layer has been proposed.
For example, as this material for forming the capacitor layer, a so-called double-sided copper clad laminate in which an FR-4 insulating base material being formed of a glass cloth impregnated with epoxy resin is used as a dielectric layer and copper foil is laminated to both sides of the base material, or a double-sided copper clad laminate comprising a dielectric layer without a skeletal material such as the glass cloth has been used.
However, in the case of using the material for forming the capacitor layer which has been conventionally supposed, it has been very difficult to make the dielectric layer thinner during the formation thereof, and consequently, it has also become difficult to increase a capacitance of the capacitor thus formed.
That is, the material for forming the embedded capacitor layer which comprises the double-sided copper clad laminate including the FR-4 base material has the glass cloth as the skeletal material within the insulating layer, so that there is a certain limit to the thickness control when the layer is made thinner. In addition, when the copper clad laminate is formed by laminating the copper foil and the FR-4 base material and then subjecting the laminate to hot press molding, a waviness shape of the glass cloth may develop on a surface of the laminate, so that it is difficult to make the layer flat completely. Even if a prepreg which uses a glass cloth as a skeletal material is used for forming an insulating layer, a limiting thickness of the prepreg is about 50 μm and it has been impossible to make the prepreg thinner than 50 μm.
Further, there has been a demand for a material which provides a capacitor having higher capacitance, therefore, an attempt to manufacture a thin double-sided copper clad laminate for forming a capacitor layer has been made by using two sheets of resin-applied copper foil each of which is copper foil having a resin layer on one side thereof and then laminating these copper foil such that respective resin layers directly contact with each other. In this method, it is easily possible to make the resin layer thinner because a skeletal material is not included within the resin layer to be used as a dielectric layer, and consequently, it is also possible to obtain an insulating layer to be used as a dielectric layer having a thickness of 20 μm or less that is substantially thinner compared to a case of the above described FR-4 base material, so that the capacitance of the capacitor can be increased to a high level such as 1 nF/cm2 to 2 nF/cm2. However, even in this method, the thickness control of the resin layer of the resin-applied copper foil has been carried out on the order of microns, so that it has been impossible to make the dielectric layer further thinner.